Electric induction heating can be used to heat electrically conductive materials. Induction heating may be used, for example, prior to forging, extrusion, rolling and other metal hot and warm forming operations. In other applications induction heating of electrically conductive workpieces can be used for heat treatment processes such as hardening, annealing, normalizing, stress relieving and tempering. Some applications require uniform heating of the entire workpiece, while other applications require heating of specific regions of the workpiece, or require heating to gradient temperatures through the workpiece such as an aluminum billet prior to an extrusion process.
As illustrated in FIG. 1(a) workpiece 90, which may be cylindrical in shape, is held in place within solenoidal induction coil 30. Support structure for holding the workpiece within the coil is not shown in FIG. 1(a). When suitable ac power is supplied to the coil, the electrically conductive workpiece is inductively heated by magnetic coupling with the generally longitudinal flux field established by the flow of ac current through the coil. When uniform heating along the length of the workpiece is desired, the workpiece is positioned in the coil so that the opposing ends of the coil overhang the opposing ends of the workpiece in the coil. The longitudinal central axis (designated X′ in FIG. 1(a)) of the coil and workpiece may be coincident as shown in the figure, and the coil is generally shaped to coincide with the longitudinal surfaces of the workpiece, or to achieve varying degrees of induced heating along the length of the workpiece. The coil overhang distance, xoh, at each end of the coil controls the shape of the flux field established in the interior overhang regions of the coil so that the flux field intensity established within the opposing ends of the workpiece provides for uniform heating along the length of the workpiece, including the opposing ends of the workpiece, as required in this example. For example a uniform longitudinal temperature T1 (graphically illustrated in FIG. 1(b)) may be achieved along the entire length L1 of the workpiece in the isothermal cross section region Riso defined between adjacent idealized isothermal dashed lines in FIG. 1(c). The overhang distance required to achieve this workpiece heating profile is affected by a number of parameters, including the outside diameter (OD) of the workpiece; the overall length of the workpiece; the workpiece's physical and metallurgical properties; coil geometry and the frequency of the ac power applied to the coil. The term “workpiece characteristics” is used to collectively describe the physical dimensions and metallurgical properties of the workpiece. Therefore different coils, each with unique characteristics, and possibly also different power supplies, are ideally used to uniformly heat workpieces of different sizes or different physical properties. However changing coils in an industrial environment to accommodate workpieces with different characteristics is time and cost ineffective. Therefore accommodations are often made to heat various sizes of workpieces in the same induction coil connected to one ac power source with varying degrees of success.
Variation in the length of a workpiece heated in a single induction coil directly impacts the coil overhang distances at each end of the coil and, consequently, the temperature distribution along the overall length of the inductively heated workpiece. For example when inductively heating a cylindrical workpiece with a relatively short overall length in an induction coil designed for uniform longitudinal heating of cylindrical workpieces with longer overall lengths, the end regions of the shorter workpiece that are exposed to greater coil overhang regions than the overhang regions for the longer workpieces will have excessive heat sources and, consequently, will be overheated relative to the central region of the shorter workpiece. For example FIG. 2(a), FIG. 2(b) and FIG. 2(c) each illustrate the same induction coil 30 used to inductively heat three workpieces having different dimensions. Workpiece 90a in FIG. 2(a) has an OD equal to OD1 and an overall length equal to L1; workpiece 90b in FIG. 2(b) has an OD equal to OD1 and a length equal to L2, which is less than length L1; workpiece 90c in FIG. 2(c) has an OD equal to OD2, which is less than outside diameter OD1 and an overall length equal to L1. As illustrated by the heated workpiece temperature distribution profiles in FIG. 2(a)′, FIG. 2(b)′ and FIG. 2(c)′, respectively for the arrangements in FIG. 2(a), FIG. 2(b) and FIG. 2(c), the coil overhang distance Xoh1 provides the desired uniform temperature distribution along the overall length of the workpiece of the particular geometry shown in FIG. 2(a), but the same coil fails to provide temperature uniformity along the length of the workpieces of different geometries in FIG. 2(b) and FIG. 2(c). Positioning the shorter workpiece in the coil non-symmetrically (FIG. 2(b)) so that the coil overhang distance at one end would be the optimal (Xoh1) would then provide the desired temperature uniformity at that end of the workpiece at the expense of intensifying overheating at the opposing end of the workpiece. Heating a workpiece with an OD less than the OD of a workpiece for which the induction coil was designed to uniformly heat results in underheating of the ends of the smaller OD workpiece due to the reduction of heat sources from the electromagnetic end effect as shown in FIG. 2(c)′ for the arrangement in FIG. 2(c).
If two workpieces have the same shape but are fabricated from materials with different physical or metallurgical properties, for example metal alloys with different electrical resistivities (ρ), using an induction coil and power supply designed to inductively heat the first of the two workpieces with an electrical resistivity of ρ1 to a uniform longitudinal temperature distribution profile will result in overheating of the ends of the second workpiece that has an electrical resistivity ρ2, which is less than ρ1, due to the electromagnetic end effect. Conversely if the second workpiece has an electrical resistivity, ρ3, which is greater than ρ1, underheating of the ends of the second workpiece will result.
An alternative approach to a single solenoidal coil with power supply connections at opposing ends of the coil is a coil with multiple power supply tap connections 80 along the length at one end of the coil as diagrammatically illustrated in FIG. 3(a) and FIG. 3(b). By selecting a power supply end tap 80 based upon the characteristics of the workpiece to be heated in the coil, the energized length of the coil, and therefore the overhang distances, can be changed to provide uniform heating of workpieces with different characteristics, such as workpiece 90a′ (utilizing end tap 80b) in FIG. 3(b), which is shorter in overall length than workpiece 90a (utilizing end tap 80a) in FIG. 3(a). There are several drawbacks to this multiple tap configuration. For example workpiece heating production time is lost when the taps are manually changed. These and other factors make a multiple tap coil arrangement disadvantageous for inductively heating a large variety of workpieces with different characteristics.
One object of the present invention is to selectively control the induced heating temperature distribution profile of electrically conductive workpieces with different characteristics in the same induction coil or combination of induction coils.
Another object of the present invention is to achieve a uniform temperature distribution profile along the overall length of electrically conductive workpieces with different characteristics in a single induction coil or combination of induction coils.
Another object of the present invention is improving the versatility of an induction heating system comprising a single induction coil and power supply by selectively controlling the induced temperature profile of electrically conductive workpieces with different characteristics in the single induction coil.